1. Field of the Invention
The present invention relates to an automatic focusing circuit which is suitable when it is used in, for example, a video camera and also relates to a video camera apparatus using the above automatic focusing circuit.
2. Description of the Related Art
A video camera in which an image pickup signal of an object is formed as an image on a CCD image pickup device and is photoelectrically converted and is subjected to a predetermined signal process and the processed signal is recorded onto a tape is generally used. Ordinarily, an automatic focusing mechanism for automatically focusing a focusing lens to an object is provided for the video camera. Please refer to U.S. Pat. No. 5,093,716 filed by the same applicant as the present invention with respect to the related art of the automatic focusing. In order to make the automatic focusing mechanism operative, a video signal is detected by an automatic focusing detecting circuit. In this instance, it is assumed that the maximum portion of high frequency components in the video signal is set to an in-focus position. The detection signal (hereinafter, refereed to as an evaluation value) is supplied to a focusing lens driving motor as a control signal. The focusing lens is moved to the in-focus position by the control signal.
FIG. 1 is a graph showing the relation between the position of the focusing lens (an axis of abscissa) and the evaluation value (an axis of ordinate). In FIG. 1, when the evaluation value is maximum, the position of the focusing lens is moved to the in-focus position. 0n the other hand, in the case where the evaluation value is low, it is judged that the present position of the focusing lens is located at an out-of-focus (blur) position, so that the focusing lens position is gradually moved to the in-focus position.
FIG. 2 is a diagram with respect to a mountain climbing control to move the focusing lens to the in-focus position. An axis of abscissa indicates the position of the focusing lens. An axis of ordinate indicates the evaluation value. For instance, when a power source of the video camera is turned on, it is detected that the focusing lens is located to the position of an evaluation value S. Information indicative of such a state is supplied to a controller. When the focusing lens is moved from the evaluation value S to, for example, an evaluation value T, the evaluation value T of the focusing lens at that time point is detected and the information is supplied to the controller. The controller compares the evaluation values S and T. In the case where the evaluation value T is larger than the evaluation value S, the focusing lens is further moved in the direction of the in-focus position. The next comparison is executed by setting the evaluation value T as a reference. Namely, the comparison between a new evaluation value U of the focusing lens which was moved to the in-focus position direction and the evaluation value T is executed. As will be also understood from FIG. 2, the evaluation value T has a value larger than the evaluation value U. Due to this, it is judged that the evaluation value is equal to the peak value. That is, the focusing lens is located at the in-focus position and the mountain climbing control is finished.
On the other hand, as operations of the automatic focusing, there are activating, in-focusing, and stand-by operations. They are classified into modes and a mode transition is performed. As kinds of modes, for example, there are an activating mode, a mountain climbing mode, a returning mode for returning to the in-focus point, a peak confirming mode, a small frame confirming mode, a wobbling mode, a stopping mode, a special processing mode, and the like.
As shown in FIGS. 3A and 3B, the evaluation value is large at the time of in-focus and is small at the time of out-of-focus. Since the evaluation value is relative, however, it is impossible to judge whether the focusing lens is located at the in-focus position or at the out-of-focus position from the absolute value of the evaluation value. Namely, an evaluation value .alpha. in the case where the focusing lens is located at the in-focus position in FIG. 3B is set to the same level as that of an evaluation value in a blur region of the focusing lens in FIG. 3A. When referring to FIG. 3, for example, in the case where a scene of whole black is being photographed, an output (evaluation value of noises) at a certain level is obtained in spite of the fact that the focusing lens is located at the in-focus position. As a result, there is a case where an erroneous operation occurs in dependence on the evaluation value of noises. On the other hand, when the focusing lens is located at the position of a large blur for a focussing position, since the focusing lens is gradually moved to the in-focus position direction, it takes a long time to obtain the in-focus state.